Organic-inorganic perovskites have recently attracted much attention due to their unique electrical, magnetic, and optical properties, as well as their excellent film processability. Layered perovskites, (RNH.sub.3).sub.2 (CH.sub.3 NH.sub.3).sub.n-1 M.sub.n I.sub.3n+1 (M=group IVB metal), naturally form a quantum-well structure in which a two-dimensional semiconductor layer of corner-sharing MI.sub.6 octahedra and an organic ammonium layer are alternately stacked. The excitons, resulting from the low-dimensionality of these semiconductor sheets, have a binding energy of several hundred milli-electron-volts, which enables strong emission even at room temperature. Their strong room temperature photoluminescence, along with significant photoluminescence wavelength tunability make the organic-inorganic perovskites attractive candidates for emitter materials in electroluminescent devices. Era et al., in Appl. Phys. Lett., V. 65, p. 676, have recently (1994) reported an electroluminescent device using the layered perovskite (C.sub.6 H.sub.5 C.sub.2 H.sub.4 NH.sub.3).sub.2 PbI.sub.4 as an emitter material. At liquid nitrogen temperature, an electroluminescent intensity of 10,000 cd/m.sup.2 was obtained at a current density of 2 A cm.sup.-2, using an applied voltage of 24 V.
Proper processing is essential in order to generate the material quality and morphology required to observe strong luminescence or to build devices. Single crystals and deposited thin films are two of the most useful forms for studies of optical and electrical properties. In general, when high-quality single crystals are required, crystal growth from the melt phase is often used. However, as a result of the organic ammonium cations, which decompose at relatively low temperature (&lt;250.degree. C.), organic-inorganic perovskites are typically made using solution chemistry techniques. Crystals resulting from solution methods are often, however, too small or of insufficient quality to be useful in device applications.
For the preparation of thin films, the spin-coating technique is suitable for processing many organic-inorganic perovskites because they are often substantially soluble in conventional organic solvents. Spin-coating can be considered a special case of solution crystal growth. It allows the formation of perovskites on a substrate, while the solvent is evaporating off. Using this method, high-quality, highly oriented layered perovskite thin films can often be obtained. However, control of film thickness, uniformity, and surface morphology is difficult using spin-coating. In addition, while simple organic ammonium salts are soluble in a range of organic solvents, including those which can dissolve the inorganic MI.sub.2 salt, for more complex organic cations, the choice of solvent becomes more limited. Furthermore, solvent techniques are not always compatible with the MI.sub.2 salt, due to problems with solubility, strong solvent coordination or the stability of the metal valence state.
Vacuum evaporation techniques have also recently been employed to grow oriented thin films of layered perovskites through a dual-source vapor deposition process. The benefits of this technique are that it is possible to precisely control the thickness and smoothness of the thin film surfaces. However, the preparation of various perovskites using different organic components is limited because each organic component easily contaminates the inside of the evaporation equipment. In addition, in some cases, the organic salt might not be thermally stable up to the temperatures required for evaporation, making this approach impractical for these systems. Even when it is possible to evaporate the organic salt, it is often difficult to balance the organic and inorganic rates, an important criterion for achieving the correct compositions of the resulting perovskite films. It is even more problematic that for each new organic-inorganic system, a re-establishment of the rates has to be carried out empirically.